Current analysis machines, as are used as a matter of routine in analytics, forensics, microbiology and clinical diagnostics, are able to carry out a multiplicity of detection reactions and analyses with a multiplicity of samples. In order to be able to carry out a multiplicity of examinations in an automated manner, various automatically operating apparatuses for the spatial transfer of measuring cells, reaction containers and reagent liquid containers are required, such as, e.g., transfer arms with a gripper function, transport belts or rotatable transport wheels, and apparatuses for transferring liquids, such as, e.g., pipetting apparatuses. The machines comprise a central control unit which, by means of appropriate software, is able to largely independently plan and work through the work steps for the desired analyses.
Many of the analysis methods used in such analysis machines operating in an automated manner are based on optical methods. Measurement systems based on photometric (e.g., turbidimetric, nephelometric, fluorometric or luminometric) or radiometric measurement principles are particularly widespread. These methods enable the qualitative and quantitative detection of analytes in liquid samples without having to provide additional separation steps. The determination of clinically relevant parameters, such as, e.g., the concentration or the activity of an analyte, is often implemented by virtue of an aliquot of a bodily fluid of a patient being mixed simultaneously or in succession with one or more test reagents in a reaction vessel, as a result of which a biochemical reaction is put into motion, which brings about a measurable change in an optical property of the test preparation.
The measurement result is, in turn, forwarded into a memory unit by the measurement system and evaluated. Subsequently, the analysis machine supplies a user with sample-specific measurement values by way of an output medium, such as, e.g., a monitor, a printer or a network connection.
Sample liquids or reagent liquids are usually transferred by means of automated pipetting apparatuses. Such pipetting apparatuses generally comprise a height-adjustable pipetting needle arranged vertically on a horizontally displaceable or swivelable transfer arm, which pipetting needle is connected to a pumping unit such that a desired volume of a liquid can be taken from a container by way of the pipetting needle, and output into a target container at a different location. Usually, the pipetting needle is displaced to a position over a liquid container with the aid of the transfer arm and then lowered into the liquid container and into the liquid contained therein. Once the desired volume has been withdrawn, the pipetting needle is driven upward and then driven to the desired target position over a liquid container, e.g., over a measurement cell, with the aid of the horizontally displaceable or swivelable transfer arm. There, the pipetting needle is lowered again, and the amount of liquid is output.
The pipetted liquid volumes lie in a range from approximately 5 to 500 microliters. Since pipetting errors or pipetting inaccuracies may lead to incorrect measurement results, very high requirements are placed on the pipetting accuracy. What is decisive here, inter alia, is the precision of the metering pump connected to the pipetting apparatus. The use of metering pumps in the form of reciprocating pumps, which are filled with a liquid or gaseous system medium and connected at the outlet opening thereof to the pipetting needle via a tubing system, is known. Negative pressure is generated by moving the pump plunger in order to suck liquid from a liquid container by means of the pipetting needle or positive pressure is generated in order to release the liquid again. Usually, such metering pumps are connected by way of a control line to a control device which, on the basis of specific control variables, is able to modify the plunger lift in accordance with the amount of volume to be pipetted.
It is problematic that a metering pump suffers from wear-and-tear over time, as result of which there are unwanted pipetting inaccuracies and, in the worst case, the abrupt outage of the metering pump and hence of the pipetting apparatus. Replacing a metering pump always requires the intervention of a user, usually even a specifically trained service technician. The unexpected outage of a metering pump, which reduces the throughput speed of the entire analysis machine and, in certain circumstances, causes a complete standstill of the analysis machine, should therefore be avoided where possible.